A fuel cell is a device in which electricity is generated by cause a fuel gas and an oxidation gas to undergo an electrochemical reaction. There has been known a fuel cell system which includes a gas supply unit which supplies fuel gas and oxidation gas to the fuel cell. Such a fuel cell system typically comprises, as a fuel gas supply unit, a supply conduit which supplies the fuel gas from a fuel gas source to the fuel cell. Then, the fuel cell system has a circulation conduit which returns discharged fuel gas discharged from the fuel cell to the supply conduit. The discharged fuel gas includes water generated by electrochemical reaction and fuel gas which is discharged from the fuel cell without having undergone electrochemical reaction. Such fuel gas contained in the discharged fuel gas is returned to the fuel cell via the circulation conduit and the supply conduit, and, accordingly, electric generation efficiency of the fuel cell system is enhanced.
In such a fuel cell system, the fuel gas in the fuel gas source is influenced by the outside environment, and the temperature of the fuel gas tends to become approximately the same as the outside temperature. Then, the fuel gas in the fuel gas source is depressurized from a high pressure state and delivered to the supply conduit, such that the temperature of the fuel gas which flows in the supply conduit becomes lower than the outside temperature. When the outside temperature is near 0° C., the temperature of the fuel gas flowing through the supply conduit, which is lower than the outside temperature, is below the freezing point of water. As moisture present in the discharged fuel gas flowing from the circulation conduit to the supply conduit is cooled by the fuel gas which below the freezing point, there is a problem that the moisture can be frozen in the supply conduit.
Japanese Patent Application Laid-Open No. 2005-332676 discloses a fuel cell system which prevents the freezing of water in a supply conduit by supplying a portion of the heat generated by electrochemical reaction to the supply conduit. The circulation conduit of the fuel cell system is provided with a gas-liquid separator by which moisture presented in the discharged fuel gas is separated therefrom and released to the environment. When the gas-liquid separator discharges the moisture contained in the discharged fuel gas to the outside, heat energy generated by the electrochemical reaction is also expelled at the same time. In a case wherein moisture is likely to be frozen in the supply conduit, the fuel cell system disclosed in Japanese Patent Application Laid-Open No. 2005-332676 controls the gas-liquid separator so as to not release the moisture contained in the discharged fuel gas and to increase the amount of the heat generated by the electrochemical reaction, the excess heat being delivered to the supply conduit. As more heat generated by the electrochemical reaction is supplied into the supply conduit, it becomes possible to warm the fuel gas delivered from the fuel gas source sufficiently to prevent freezing in the supply conduit.
However, when the amount of heat generated by the electrochemical reaction is small, the fuel cell system disclosed in Japanese Patent Application Laid-Open No. 2005-332676 cannot sufficiently warm the fuel gas flowing from the fuel gas source. There therefore remains a possibility that the moisture present in the discharged fuel gas will be cooled by the fuel gas to the extent that it will be frozen in the supply conduit.